The joint torque generated by twitch contraction is increased after brief conditioning contractions performed voluntarily (17,24,40) and percutaneously evoked (22,25,31). This phenomenon is collectively called “twitch potentiation” (TP). Recently, it has been shown that a conditioning contraction for inducing TP enhances subsequent dynamic joint performance with maximal voluntary effort, such as dynamic joint torque and joint power development (3,21,23). For example, Miyamoto et al. (23) reported that the maximal voluntary isokinetic concentric torque was significantly increased after a 6-s maximal voluntary isometric contraction (MVC). These observations support Sale’s (34) hypothesis that activity-dependent potentiation can also occur in the joint performance with maximal voluntary effort. However, the potentiation of the voluntary joint performance in the previous studies (3,23) failed to occur immediately after the conditioning MVC, despite the occurrence of maximal TP. Our previous study has shown that EMG activities of agonist muscles, during the conditioning MVC and during the maximal concentric torque production immediately after the conditioning contraction, were decreased without changes in the M-wave amplitude of respective muscles (23). These results strongly suggest that central fatigue occurred during the conditioning contraction and that, consequently, it could attenuate the enhancement of the subsequent voluntary concentric torque. Indeed, in the two previous studies (3,23), the voluntary joint performance was further increased after longer recovery intervals (approximately 1 to 3 min). Similarly, Gossen and Sale (15) have shown that, even in the presence of TP, no significant increase of angular velocity or peak power during dynamic knee extension was observed within 1 min after a 10-s MVC. Thus, it is expected that reducing the central fatigue during conditioning contractions while keeping greater potentiative effect leads to greater enhancement of dynamic joint performance.
One approach to prevent the central fatigue that occurs during the 6-s conditioning MVC is to shorten its duration. According to the previous results mentioned above (3,15,23), it is expected that a duration of 5–6 s or less of conditioning MVC can be effective for enhancing the subsequent dynamic joint performance with maximal efforts. However, the effect of the duration of conditioning MVC on the subsequent joint performance remains unclear. Although a short duration of the conditioning MVC can avoid the occurrence of the central fatigue, previous studies showed that conditioning contraction can produce only small TP when the duration is too short (40). Moreover, to increase voluntary dynamic joint performance with the maximal effort, it is suggested that large TP with sufficient phosphorylation of myosin regulatory light chain is required (3,23). Taken together, the two factors (i.e., more potentiation and less fatigue) should be considered when one sets the optimal duration of the conditioning contraction to enhance voluntary dynamic performance. Therefore, the present study aimed to examine the effect of three types (short, moderate, and long duration) of the conditioning MVC on the maximal voluntary concentric torque.
Another possible approach to avoid or minimize the central fatigue is the use of percutaneous neuromuscular electrical stimulation (NMES) as a conditioning contraction because NMES bypasses volition for muscle contractions (11). Previous studies have compared the extent of TP elicited after voluntary contraction and NMES (18,22,23). Our recent study demonstrated that a 10-s NMES with a relatively low stimulation frequency (20 Hz) at an intensity of 60% MVC elicited greater TP compared with a 10-s voluntary contraction performed at an identical torque level and that the former induced as much extent of TP as a 10-s MVC did (22). Namely, NMES with a low stimulation frequency can induce sufficient TP. However, when NMES is applied as a modality for enhancing voluntary joint performance, the duration of NMES with low stimulation frequency should be taken into consideration because a longer duration of NMES should elicit greater peripheral fatigue as well as TP. Therefore, we first examined the effect of duration of NMES with low frequency on the extent of TP and then investigated the influence of NMES on the subsequent dynamic joint performance with maximal voluntary effort.
The main purpose of the present study was to examine whether the use of shorter duration MVC or NMES at a low stimulation frequency can be a modality for enhancing effectively maximal voluntary concentric torque. In the present study, we tested this by measuring twitch responses and EMG signals as well as maximal voluntary concentric torque to consider from the viewpoint of both potentiative effect and fatigue.
The present study consisted of two experiments. Each experiment was performed on separate days to avoid the effect of muscle fatigue. The main purpose of the first experiment (experiment 1) was to examine the effect of duration of conditioning 20-Hz NMES with uncomfortable stimulation intensity on the extent of TP and to determine the optimal duration for inducing greater TP. The reason for assessment by the twitch responses rather than the voluntary dynamic joint performance in experiment 1 was that the recording of twitch responses and EMG permit the motivation of the subjects and the magnitude of drive from the CNS to the muscles to be removed as modifiers of performance output. In the second experiment (experiment 2), the magnitudes of enhancement of maximal voluntary concentric torque after MVC with three different durations or NMES with optimal duration based on the result of experiment 1 were examined. The interval period between experiments 1 and 2 was at least 2 wk (14–19 d).
Thirteen healthy male subjects (26.7 ± 2.9 yr, 173.8 ± 4.6 cm, 69.2 ± 5.2 kg, mean ± SD) were recruited for this study. Before participation, all subjects were fully informed of the experimental procedures, possible risks, and their right and gave their written consent to participate. The purpose and hypothesis of the study were not explained to the subjects until termination of experiment 2 measurements to exclude any potential bias that might affect the results. They were asked to refrain from strenuous exercising for 24 h before the testing sessions and not to consume caffeinated drinks on the day of testing. This study was approved by the ethics committee on human research of Waseda University and performed in accordance with the Declaration of Helsinki.
General Experimental Setup
Twenty-four to forty-eight hours before each experiment, a familiarization session was conducted. The purpose of this session was to familiarize the subjects with the maximal voluntary torque production of isometric and isokinetic concentric knee extension on a dynamometer (CON-TREX; CMV AG, Dübendorf, Switzerland). The subject began the familiarization session with a warm-up exercise by performing several submaximal isometric knee extensions and then performed two MVCs for approximately 3 s, with a rest period of 2 min. In the familiarization session and all experiments, each subject was seated on the dynamometer seat with the hip flexed at 80°. The subject was tightly secured to the dynamometer seat by using two crossover seatbelts and a waist harness, and the lever arm of the dynamometer was attached 2–3 cm above the lateral malleolus with a strap. The rotation axis of the knee was aligned to the motor’s axis.
Surface EMG signals were obtained from the vastus medialis (VM), vastus lateralis (VL), rectus femoris (RF), and biceps femoris muscles. After shaving, rubbing with sandpaper, and cleaning with alcohol, preamplified bipolar surface electrodes (1 × 10 mm, 10-mm interelectrode distance) with band-pass filtering between 20 and 450 Hz (DE-2.1; DelSys, Monmouth Junction, NJ) were placed longitudinally over the belly of the muscles. The reference electrode was placed over the left patella. The torque and EMG data were simultaneously sampled at 4 kHz by using a 16-bit analog-to-digital converter (PowerLab/16SP; ADInstruments, Sydney, Australia) for later analysis.
A specially modified multifunctional stimulator (Atrjum, Saitama, Japan) was used to elicit the quadriceps femoris muscle contractions as conditioning contractions by NMES. On the basis of the result of our recent study (22), the stimulation frequency was set at 20 Hz to induce large TP and minimize the occurrence of peripheral fatigue. In the present study, the stimulation intensity of NMES was adjusted to the intensity within mild-to-moderate pain and less than severe pain for each subject because uncomfortable and painful stimulation can reduce the motivation of the subjects and/or can cause reflex cortical activity (8), which may result in the central fatigue. Two 15 × 5-cm electrodes were positioned on the anterior aspect of the thigh; one electrode was placed 10 cm above the upper border of the patella, and the other was positioned on the proximal portion of the thigh. The stimulation intensity used as NMES conditioning contractions was adjusted by applying three to five bouts of 3-s tetani with 250-μs biphasic rectangular wave pulses. The pulse duration of NMES was chosen to avoid the activation of sensory fibers and to minimize discomfort because a longer pulse duration is sometimes painful and consequently can cause reflex contractions (8) and because sensory nerve fibers are easily activated by electrical stimulation of pulse duration longer than 500 μs (29).
To evoke quadriceps femoris muscle twitches, the femoral nerve was stimulated transcutaneously by using the cathode (2 × 2 cm) placed on the femoral triangle. The anode (8 × 5 cm) was positioned midway between the superior aspect of the greater trochanter and the inferior border of the iliac crest. Single square-wave pulses of 1-ms duration were delivered from a high-voltage electrical stimulator (SEN-3301; Nihon Kohden, Nakano-ku, Tokyo, Japan) with a specially modified isolator (SS-1963; Nihon Kohden). Supramaximal stimulus intensity was determined before the testing by increasing the voltage until the twitch torque and M-wave amplitude reached a plateau and then set to 20% above the maximum for the experimental measurements.
Subjects initially warmed up by performing several knee extension exercises on the dynamometer with the knee flexed at 70°. At first, the electrodes for NMES were placed, and the stimulation intensity was determined for each subject. Then, the subjects rested for approximately 20 min during the placement of the electrodes for femoral nerve stimulation and EMG recording. This resting interval was sufficient for potentiative effects of the warm-up and the NMES to disappear (23,24,40). Before each conditioning contraction, the twitch responses to the femoral nerve stimulation were recorded (PRE). Thereafter, a conditioning contraction for 3, 5, or 10 s was performed through NMES. Twitch responses to the femoral nerve stimulation with the same intensity and duration as in PRE were then recorded during the recovery interval in the following sequence: immediately after (3 s) the conditioning contraction and then every 1 min thereafter until the 5-min point (POST). At least 10-min rest was provided between the end of each trial and the beginning of the next one, and the subsequent measurements were performed only when twitch peak torque did not differ by more than 5% from the initial value. For each subject, this protocol was repeated for each conditioning contraction in a randomized order.
The experimental setup was identical with that in experiment 1 except in terms of the knee joint angle. Similarly in experiment 1, each subject initially performed several muscle contractions voluntarily and evoked through NMES as warm-up exercises, and approximately 20-min rest was provided for placing the electrodes. In experiment 2, four types of conditioning contraction were used: 3-s MVC, 5-s MVC, 10-s MVC, and 5-s NMES at 20 Hz. For the NMES trial, the duration of 5 s was chosen according to the result of experiment 1. In each trial, before performing the conditioning contraction, the responses to one isometric single twitch (PREbefore) and three maximal concentric contractions followed 3 s later by one isometric single twitch (PREafter) were recorded (Fig. 1). In concentric contractions, the range of motion of the knee was between 105° and 10° (0° = full extension), and the angular velocity of the dynamometer was set at 210°·s−1. The selection of this velocity was for the purpose of elucidating muscle behavior in a ballistic jumping exercise in which the knee joint rotates near this angular velocity (a result from a preliminary experiment). The last stimulation was used to examine the possible potentiative effect by a set of three maximal isokinetic concentric contractions. After these control recordings, one of the conditioning contractions was carried out. In each trial, the tests which consisted of one twitch and three maximal concentric contractions were performed at instants as follows: immediately (3 s) after the conditioning contraction and 1, 3, and 5 min thereafter (Fig. 1). The stimulation before the concentric contractions was used to quantify the extent of TP during the dynamic contractions. A minimum of 15-min rest was provided between the end of each test and the beginning of the next one. For each subject, the order of four conditioning contractions was randomized.
Peak torque, contraction time (time to peak torque), and half relaxation time of twitch and the peak-to-peak amplitude and duration of the M-wave of VM, VL, and RF muscles before and after the conditioning contractions were calculated from the evoked twitch torque and EMG data. Regarding the voluntary concentric contraction, the data in which the peak torque was the highest among three contractions per each set were adopted for analysis. The peak torque and the associated root mean square values of EMG (RMS-EMG) of all muscles were computed. Calculation of the peak torque during concentric contractions included the correction for gravity and passive torque. RMS-EMG values were calculated during a 200-ms period before the occurrence of peak torque. Relative changes in all the parameters were obtained according to the following formula: PREafter or POST value / PREbefore value × 100 (%). For the conditioning contraction data, the average torque values were computed for the first and last 1-s periods with a steady state of each conditioning contraction, and the corresponding RMS-EMG of all muscles was calculated for the periods in the 3-, 5-, and 10-s MVC trials.
The intra- and interday variability of peak torque values of twitch and maximal voluntary concentric contractions of the quadriceps femoris muscle were assessed. The intraclass correlation coefficient values were 0.94 and 0.90 in the twitch peak torque and maximal voluntary concentric torque, respectively, for the intraday comparison and 0.92 and 0.87, respectively, for the interday comparison. Moreover, the coefficients of variance for the repeated measures were 1.9% ± 0.9% and 2.8% ± 1.2% for peak torque values of twitch and maximal voluntary concentric contractions, respectively. Therefore, the parameters can be considered reproducible. For all the parameters of twitch and maximal voluntary contractions, separate two-way ANOVAs (conditioning contraction type × time) with repeated measures were used. When significant interaction was observed, additional one-way ANOVAs with Dunnett and Tukey post hoc tests were performed to determine whether significant differences existed between PRE and POST for each conditioning contraction type or between conditioning contraction types at each time point, respectively. In addition, for the conditioning contraction data, paired t-tests were used to assess the statistical difference in the RMS-EMG value of each muscle between the first and last 1-s periods during conditioning contractions. The significance level for all comparisons was set at P < 0.05. The statistical analyses were performed by using statistical software (PASW Statistics 18; SPSS Japan, Tokyo, Japan). All data are expressed as means ± SD.
Torque output during 20-Hz NMES corresponded to a level of 58.1% ± 8.8% (49.8%–64.8%) MVC. No significant difference was found across all trials in the evoked twitch contraction parameters measured before the conditioning contractions. For POST data, two-way ANOVA demonstrated a significant interaction (conditioning contraction type × time) for the twitch peak torque. Follow-up ANOVAs and post hoc comparisons revealed that the extent of TP after a 3-s NMES was significantly smaller than those after 5- and 10-s NMES (Fig. 2). On the other hand, although a 10-s NMES tended to induce slightly greater TP than a 5-s NMES, there was no significant difference in the extent of TP between the 5-s and 10-s NMES trials. For the contraction time and half relaxation time data, there were no significant main effect of the conditioning contraction type and time and no interaction (conditioning contraction type × time). Similarly, the M-wave amplitude and duration of each muscle were not influenced by the conditioning contractions in all trials. According to these results, the duration of the NMES trial in experiment 2 was determined to be 5 s.
At PREbefore, there was no significant difference in the parameters among the four trials, indicating that a 15-min recovery period was sufficient for restoration of twitch torque from potentiation. For all of the twitch torque data, the two-way ANOVA revealed that the condition × time interaction was significant. Further analyses revealed that, in all trials, twitch peak torque after a set of three maximal concentric contractions (PREafter) was significantly increased, indicating that a set of three concentric contractions induced TP (Fig. 3). Regarding the extent of TP after the conditioning contractions, the extent of TP in the 3-s MVC trial was significantly smaller compared with those in the 5-s MVC, 10-s MVC, and 5-s NMES trials, although there was no significant difference among the 5-s MVC, 10-s MVC, and 5-s NMES trials (Fig. 3). For the M-wave amplitude and duration of all muscles, there was no significant main effect of the conditioning contraction type and time and no interaction (conditioning contraction time × time).
Maximal voluntary concentric contraction.
Figure 4 shows the changes in maximal voluntary concentric torque after the conditioning contractions. Two-way ANOVA revealed a significant conditioning condition type × time interaction. Follow-up ANOVAs and post hoc comparisons revealed that, in the 3-s MVC trial, the maximal voluntary concentric torque did not significantly change after the conditioning contraction. In the 5-s MVC trial, a significant change was not observed immediately after the conditioning MVC, whereas the maximal voluntary concentric torque significantly increased at 1 and 3 min after MVC (106.6% ± 2.3% and 107.2% ± 2.6% of PRE value, respectively, P < 0.05). After a 10-s conditioning MVC, the maximal voluntary concentric torque significantly decreased immediately and 1 min after the conditioning MVC (93.9% ± 3.2% and 97.4% ± 2.2% of PRE value, respectively, P < 0.05) and returned to the baseline at 3 and 5 min after the MVC. In contrast, a 5-s NMES at 20 Hz significantly enhanced immediately after the conditioning contraction (105.1% ± 2.2% of PRE value) as well as the 1- and 3-min points (107.5% ± 3.3% and 107.8% ± 2.7%, respectively). At the time immediately after the conditioning contraction, the enhancement of maximal voluntary concentric torque in the 5-s NMES trial was significantly greater than those in the MVC trials, and the relative torque values of the 3-s MVC and 5-s MVC trials were significantly greater than those of the 10-s MVC trials (P < 0.05). At 1 min after MVC, the enhancement in the 5-s NMES trial was significantly greater than those in the 3-s MVC and 10-s MVC trials (P < 0.05). Moreover, at 3 min after MVC, the enhancement in the 5-s MVC and 5-s NMES trials was significantly larger than that in the 3- and 10-s MVC trials (P < 0.05). On the other hand, there was no significant difference in the maximal voluntary concentric torque among trials. The RMS-EMG values during maximal voluntary concentric contractions were decreased immediately after the conditioning contraction in the VL and RF muscles of the 5-s MVC trial (93.1% ± 3.3% and 91.7% ± 3.2% of PRE, respectively) and in the VM, VL, and RF muscles of the 10-s MVC trial (93.5% ± 4.1%, 87.2% ± 5.1%, and 84.8% ± 5.5% of PRE, respectively) (P < 0.05).
EMG activity during conditioning contraction.
For the RMS-EMG values of the first and last 1-s periods during the conditioning contractions of each muscle in MVC trials, a significant conditioning condition type × time interaction was observed. In the 3-s MVC trial, there was no significant difference between the two periods for all muscles, whereas the EMG activities of the VL and RF muscles in the 5-s MVC trial and those of the VM, VL, and RF muscles in the 10-s MVC trial were significantly lower during the last period (P < 0.05).
The major finding of this study is that an approximately 5-s conditioning contraction performed voluntarily and through 20-Hz NMES with high intensity can be a modality to enhance dynamic voluntary joint performance. To the best of our knowledge, this is the first study to demonstrate that conditioning contraction by NMES as well as MVC can enhance the maximal voluntary concentric torque, with consideration for both potentiative effect and fatigue. In addition, whereas the 5-s conditioning MVC enhanced the maximal voluntary concentric torque only 1 and 3 min after, 5-s NMES at 20 Hz produced the corresponding effect immediately after and provided further increases at 1 and 3 min after. This implies that the NMES procedure used here has a more immediate effect as compared with the isometric MVC conditioning contraction. Furthermore, the current study indicates that short-duration MVC (∼3 s) cannot be a sufficient stimulus to enhance the dynamic performance with maximal voluntary effort.
The primary mechanism responsible for activity-dependent potentiation including TP is considered to be the myosin regulatory light chain phosphorylation due to prior conditioning contraction (26,28), which causes individual myosin heads to swing out from the myosin backbone, thereby bringing the actin binding site of the myosin head in closer proximity to the actin filament (35). This permits a faster rate of engagement of crossbridges with no change in the rate of dissociation, which would result in more crossbridges in the force- generating state during contraction at a submaximal level of activation (i.e., twitch contraction, tetanic contractions at a low stimulation frequency, and a few trains at high frequency) (19).
Effects of potentiation and fatigue on voluntary performance in MVC trials.
In the 5-s MVC trial, the extent of TP was maximal immediately after the conditioning contraction, whereas enhancement of voluntary concentric torque occurred 1 min after the conditioning MVC. This result is in line with those of previous studies reporting that the joint power against a given load (3) and peak torque during maximal isokinetic concentric contraction (23) were enhanced with proper recovery time (>1 min) after a 6-s conditioning MVC. In contrast, Gossen and Sale (15) have reported that angular velocity and peak power during dynamic knee extension was not increased after a 10-s MVC. There are two possible factors responsible for these discrepancies: i) the duration of the conditioning contraction and ii) the recovery interval between the end of the conditioning contraction and the start of the voluntary performance measurement.
The duration of conditioning contraction affects the relative amount of potentiative effect and fatigue produced. The extent of TP of the quadriceps femoris muscle was increased with duration of the conditioning MVC up to 5 s (Fig. 3). On the other hand, fatigue should also be increased with duration of the conditioning MVC. During sustaining MVC, motor unit discharge rates decrease, and some motor units with high recruitment threshold cease to discharge within seconds (16), suggesting a possibility of the occurrence of greater fatigue with increased duration of the conditioning MVC. Indeed, greater reductions of EMG activities during conditioning MVCs were observed with increasing duration of the conditioning MVCs. The significant decreases in the EMG activities during MVCs without changes in the M-wave amplitude imply the presence of the central fatigue (11,33). Taken together, the delayed onset of the enhancement of maximal voluntary concentric torque in the 5-s MVC trial and its absence in the 10-s MVC trial would be explained by the coexistence of the dual effects by potentiation and fatigue (4,13). These observations of the present study support the hypothesis put forward by Sale (34) that voluntary performance can be transiently enhanced when fatigue disappears faster than the potentiative effect decays after the conditioning contractions.
When a shorter duration (3 s) of the conditioning MVC was performed to reduce the central fatigue, the maximal voluntary concentric torque was only marginally increased, but it was not statistically significant. This is not due to fatigue that should have been smaller than in the 5-s MVC trial but due to the smaller extent of potentiative effect (Fig. 3). On the basis of these results with previous findings that a 6-s conditioning MVC enhanced voluntary dynamic performance (3) and maximal voluntary concentric torque (23) after >1-min rest, it is suggested that, when an MVC is used as a conditioning contraction, the duration of approximately 5 to 6 s is optimal for deriving a large benefit in maximal voluntary dynamic torque, if a proper recovery interval is given after the conditioning MVC.
Effect of NMES on TP and voluntary performance.
In experiment 1, the extent of TP after a 3-s NMES at 20 Hz was significantly smaller than that after 5-s and 10-s NMES at 20 Hz, whereas there was no significant difference between the 5-s and 10-s NMES trials. Mettler and Griffin (20) examined the effects of stimulation frequency, train duration, and pulse number of stimulated conditioning contraction on the extent of TP in the human adductor pollicis muscle and concluded that, across stimulation frequencies ranging from 15 to 50 Hz, the extent of TP depended on the pulse number, not on the stimulation frequency, when the stimulated conditioning contractions were carried out at a given stimulation intensity (i.e., an identical number of recruitment of muscle fibers). Considering this result with the present finding, it is likely that the upper limit of the pulse number for determining the extent of TP in the human quadriceps femoris muscle exists between 60 (20 Hz for 3 s) and 100 (20 Hz for 5 s) pulses. In contrast, longer duration of NMES (i.e., a greater number of the pulse) would elicit further peripheral fatigue without increasing the extent of TP. Thus, in experiment 2, the duration of 5 s was chosen for the purpose of inducing greater TP and less fatigue.
Although, in experiment 2, the target torque level of the conditioning contraction was lower in the NMES trial (approximately 60% MVC) than in the MVC trials, the extent of TP after 5-s NMES at 20 Hz was comparable to those after 5-s and 10-s MVCs (Fig. 3). This observation seems to be puzzling because the extent of TP is influenced by the number and type of muscle fibers recruited during conditioning contractions (17,25,40). However, the difference in target torque levels does not necessarily correspond with that in the number of muscle fibers recruited. According to the torque–stimulation frequency relationship of human in vivo muscles, maximal isometric torque of the quadriceps femoris muscle is commonly obtained at frequencies of approximately 50–60 Hz (1,6) similarly to other muscles (10,12). On the other hand, the torque level during 20-Hz stimulation corresponds to approximately 60% of maximal torque. This observation was confirmed in 3 of 13 subjects of the present study (Fig. 5), in whom the torque–stimulation frequency relationship was assessed by stimulating the quadriceps femoris muscle through NMES at 70° of knee angle. These findings indicate that the number of muscle fibers recruited during the conditioning contraction would be almost the same in both the 20-Hz NMES at 60% MVC level and the MVC trials, resulting in the comparable extent of TP regardless of different target torque levels of the conditioning contractions between the 5-s or 10-s MVC and the 5-s NMES at 20 Hz trials in the present study.
When NMES was used as a conditioning contraction, maximal voluntary concentric torque was increased immediately after the conditioning contraction unlike in the 5-s MVC trial. This is because NMES can bypass the CNS during muscle contractions and consequently reduce the central fatigue while keeping greater potentiative effect. However, the maximal voluntary concentric torque was further increased 1 min after the conditioning contraction despite decreasing TP. This could be due to the larger peripheral fatigue immediately after the conditioning NMES.
Possible reasons for enhancement of maximal voluntary concentric torque.
A mention should be made of the possible reasons for the potentiation of maximal voluntary concentric torque. The mechanism for the activity-dependent potentiation is considered to be a larger number of crossbridges in the force-generating state during contraction, due to phosphorylation of the regulatory light chain of myosin (37). Until recently, it has been generally accepted that this plays out nicely for submaximal contractions but has almost no role in Ca2+-saturated conditions such as maximal contraction evoked with high stimulation frequency (38,39). Nevertheless, the present and previous (3,21,23) studies have shown that the joint performances with maximal efforts were potentiated after conditioning contractions. This could be because the activation level of the muscle fibers recruited during the maximal voluntary dynamic contractions is actually submaximal.
As mentioned previously, almost maximal isometric torque of the quadriceps femoris muscle has been shown to be obtained at frequencies of approximately 50–60 Hz during stimulation with certain intensity (Fig. 5). However, the torque–stimulation frequency relationship for isometric contractions cannot simply be extrapolated to dynamic contractions. De Haan (9) showed in the rat medial gastrocnemius muscle that the stimulation frequency required to attain maximal activation is higher in isovelocity shortening contraction than in isometric contraction. We also confirmed the rightward shift of the normalized torque–frequency relationship in concentric contraction at 210°·s−1 compared with that in isometric contraction at 70° of knee flexion (Fig. 5). This rightward shift would be due to the fact that, during rapid shortening contraction, the dissociation of crossbridge is accelerated (7,36), and this leads to a decrease in the relative activation level at a given [Ca2+]. Thus, higher stimulation frequency is required to maintain maximal activation in concentric contraction. As far as we know, the average discharge frequency of motor units during isometric MVC of the human quadriceps femoris muscle was approximately 50 Hz (30,32), which matches the stimulation frequency required to attain the maximal activation of the quadriceps femoris muscle. Therefore, it is possible that the activation level during isometric MVC is almost “truly maximal.” On the other hand, there is no available data regarding the motor unit discharge frequency during dynamic contraction, especially as fast as 210°·s−1, because it is impossible to measure the motor unit discharge frequency during voluntary dynamic contraction because of change in diameter of muscle fibers and corresponding change in wave form of motor unit action potentials. However, it is unlikely that the motor units discharge at 150 Hz or more (which corresponds to the stimulation frequency to attain the maximal activation in the dynamic condition of the additional experiment) only in voluntary dynamic contractions because some previous studies reported in humans in vivo that very high discharge rates up to 250 Hz could occur at the initial phase of a ballistic contraction, although the short-interval bursts lasted only for four to five pulses and such instantaneous discharge always dropped at the later phase of the ballistic contraction (5,16). Thus, the activation level during concentric MVC would be submaximal. Taking these observations into account together with the fact that the potentiative effect is large when the activation level is low (26,28), concentric contraction likely gets more of a benefit from the potentiative effect as compared with isometric contraction, despite maximal voluntary efforts in both contractions.
Other factors such as muscle temperature and subjects’ training status, which could affect muscle contractile properties (14,17), were not explored in the present study. However, muscle temperature has been shown not to differ after a 6-s MVC (2). In addition, the effect of temperature on each protocol was controlled by randomizing the order for each subject. Thus, we believe that the effect of the muscle temperature on the present results is negligible, if any. Regarding the training status of the subjects, previous studies have shown that it could affect the manifestation of potentiation and/or fatigue after the conditioning contraction (17,27). Furthermore, TP is more prominent in Type II muscle fibers, whereas muscles with a predominance of Type II fibers are fatigable (17,26). Therefore, although it is likely that the training status of the subjects (i.e., longitudinal resistance and/or endurance training intervention to overcome muscle fatigue) could influence the extent and time course of the enhancement of voluntary dynamic joint performance, no study has examined these aspects with the assessment of TP or phosphorylation of myosin regulatory light chain. Future studies are warranted to reveal whether the training status of the subjects could influence the enhancement of voluntary dynamic joint performance.
From the practical viewpoint, we recommend including high-intensity short-duration muscle contraction during warm-up procedures for sports activities. Namely, although submaximal contractions have so far been used as warm-up procedures to reduce fatigue, the current results indicate that high-intensity short-duration contractions can be a more effective warm-up modality for high-power activities. In addition, considering the present findings with our previous results (22,25), we can recommend the use of NMES with proper duration, intensity, and stimulation frequency as a more immediately effective warm-up modality for explosive sports activities. Furthermore, the current findings can provide an evidence for the effectiveness of complex training, which has been defined as combining plyometric and weight resistance exercises in the same training session, and help to establish proper protocols (i.e., rest interval between plyometric and weight resistance exercises) in this training style.
In conclusion, this study revealed that when the isometric MVC was used as a conditioning contraction, the maximal voluntary concentric torque was enhanced only 1 and 3 min after a 5-s conditioning MVC and that the maximal voluntary concentric torque was significantly increased not only 1 and 3 min after but also immediately after a 5-s NMES at 20 Hz. These results suggest that approximately 5-s conditioning contractions performed with isometric MVC and through 20-Hz NMES with high intensity can be a modality to enhance dynamic voluntary joint performance, with the latter having a more immediate effect, and that short-duration MVC (∼3 s) cannot be a sufficient stimulus to enhance the dynamic performance with maximal voluntary effort.
This work was partly supported by the Grant-in-Aid for Young Scientists (B, #22700629) from the Japan Society for Promotion of Science and by the Waseda University Global COE Program: “Sport Sciences for the Promotion of Active Life.”
The authors report no conflict of interest.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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